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// Michele Signore 1, * , Mariachiara Buccarelli 1, * , Emanuela Pilozzi 2 , Gabriele De Luca 1 , Marianna Cappellari 1 , Maurizio Fanciulli 3 , Frauke Goeman 3 , Elisa Melucci 3 , Mauro Biffoni 1, § , Lucia Ricci-Vitiani 1, § 1 Department of Hematology, Oncology and Molecular Medicine, Istituto Superiore di Sanita, Rome, Italy 2 Department of Clinical and Molecular Medicine, Sant'Andrea Hospital, University La Sapienza, Rome, Italy 3 Scientific Direction, Regina Elena National Cancer Institute, Rome, Italy * These authors have contributed equally to this work § These two authors shared senior authorship Correspondence to: Michele Signore, email: michele.signore@iss.it Lucia Ricci-Vitiani, email: lriccivitiani@yahoo.it Keywords: colorectal cancer stem-like cells, kinase inhibitors, Chk1, DNA damage, phospho-proteomics Received: October 13, 2015 Accepted: May 13, 2016 Published: June 6, 2016 ABSTRACT Colorectal cancer (CRC) is one of the most common and lethal cancers worldwide. Despite recent progress, the prognosis of advanced stage CRC remains poor, mainly because of cancer recurrence and metastasis. The high morbidity and mortality of CRC has been recently ascribed to a small population of tumor cells that hold the potential of tumor initiation, i.e. cancer stem cells (CSCs), which play a pivotal role in cancer recurrence and metastasis and are not eradicated by current therapy. We screened CRC-SCs in vitro with a library of protein kinase inhibitors and showed that CRC-SCs are resistant to specific inhibition of the major signaling pathways involved in cell survival and proliferation. Nonetheless, broad-spectrum inhibition by the staurosporin derivative UCN-01 blocks CRC-SC growth and potentiates the activity of irinotecan in vitro and in vivo CRC-SC-derived models. Reverse-Phase Protein Microarrays (RPPA) revealed that, albeit CRC-SCs display individual phospho-proteomic profiles, sensitivity of CRC-SCs to UCN-01 relies on the interference with the DNA damage response mediated by Chk1. Combination of LY2603618, a specific Chk1/2 inhibitor, with irinotecan resulted in a significant reduction of CRC-SC growth in vivo , confirming that irinotecan treatment coupled to inhibition of Chk1 represents a potentially effective therapeutic approach for CRC treatment.

Epigenetic alterations, including dysregulated DNA methylation and histone modifications, govern the progression of colorectal cancer (CRC). Cancer cells exploit epigenetic regulation to control cellular pathways, including apoptotic and metastatic signals. Since aberrations in epigenome can be pharmacologically reversed by DNA methyltransferase and histone deacetylase inhibitors, epigenetics in combination with standard agents are currently envisaged as a new therapeutic frontier in cancer, expected to overcome drug resistance associated with current treatments. In this study, we challenged this idea and demonstrated that the combination of azacitidine and romidepsin with IFN-α owns a high therapeutic potential, targeting the most aggressive cellular components of CRC, such as metastatic cells and cancer stem cells (CSCs), via tight control of key survival and death pathways. Moreover, the antitumor efficacy of this novel pharmacological approach is associated with induction of signals of immunogenic cell death. Of note, a previously undisclosed key role of IFN-α in inducing both antiproliferative and pro-apoptotic effects on CSCs of CRC was also found. Overall, these findings open a new frontier on the suitability of IFN-α in association with epigenetics as a novel and promising therapeutic approach for CRC management.